Comprehensive simulation insights into molten pool dynamics, stress, and temperature fields of vacuum electron beam welded CoCrFeMnNi HEA/SA203 joints

This study joins CoCrFeMnNi HEA/SA203 steel dissimilar materials using vacuum electron beam welding (VEBW), achieving a joint strength of 87 % of the base material. However, despite this high mechanical strength, the joint is compromised by pronounced morphological undulations along the HEA-side fusion line, posing potential risks. Through a validated multi-physics simulation, we demonstrate that this defect is not due to compositional variations but is a direct consequence of the intrinsic thermophysical property mismatch between the materials. The HEA's poor thermal conductivity and high molten-state viscosity create severe asymmetric thermal-flow fields, establishing a dynamic instability within the molten pool. This instability is characterized by the convergence of stratified vortices and a periodic, high-velocity mass exchange, which is identified as the direct mechanism responsible for sculpting the irregular fusion line. The simulation results demonstrate excellent agreement with experimental microstructures and morphologies, validating the proposed mechanism. This work provides a fundamental understanding of the molten pool dynamics governing defect formation in dissimilar metal welding involving HEAs, offering a theoretical basis for mitigating such defects and advancing the reliable application of HEA-based cryogenic components.